CVE-2021-29991

8.1 highEPSS 0.9%

High (8.1) in mozilla. EPSS 0.9% chance of exploitation in the next 30 days.

Firefox incorrectly accepted a newline in a HTTP/3 header, interpretting it as two separate headers. This allowed for a header splitting attack against servers using HTTP/3. This vulnerability affects Firefox < 91.0.1 and Thunderbird < 91.0.1.

Severity

CVSS 3.1

8.1 high

CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:H/I:H/A:N

EPSS: 0.9% chance of exploitation in the next 30 days; higher than 59% of CVEs (scores of 2026-10-31)
CWE: CWE-444
Published: 2021-11-03
Modified: 2026-06-17
Record from: the NVD, synced 2026-10-19

Affected

mozilla:firefox
mozilla:thunderbird

References

https://bugzilla.mozilla.org/show_bug.cgi?id=1724896
https://www.mozilla.org/security/advisories/mfsa2021-37/
https://bugzilla.mozilla.org/show_bug.cgi?id=1724896
https://www.mozilla.org/security/advisories/mfsa2021-37/

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